Especially when you get into 'marginal' flow issues such as partly fouled Hx, etc. the internal strainer will have more 'open flow' & less resistance than an 'external' ... just a function of the additive open area of all the 'holes'. Hx function is based on mass (amount of) flow, an external strainer due to high resistance will be passing less mass of water due to the pressure drop. Something to think about if/when you are lugging an engine at WOT and when in warm water.

Check the flow rate vs. differential pressure on each type of strainer and then match with the mass flow rate vs. temperature, etc. of your Hx thermal profile before you make a firm decision. You'll probably find that the 'external' has 1/3 or less the flow rate (per pounds per square inch differential pressure) of the Internal strainer. If you cant find the data just measure or estimate the "open area" (number of holes times area of each hole) of each.